Mass spectrometry



July 12, 1949.

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MASS SPECTROMETRY Harold W. Washburn, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application April 28, 1945, Serial No. 590,917

(Cl. `Z50-4114) 8 Claims.

1 `This invention is concerned with mass specprovides improvements in methods gaseous molecules and propelling the beam into an analyzer chamber, to the end that the stability `of spectra and the accuracy and rapidity of analysis is improved.

A mass spectrometer is essentially an apparatus` for producing ions and sorting them according to the ratio of their mass to their charge, i. e. Aaccording to their specic mass. A sample mixture, is ionized (for example, by electron bombardment) and the resulting ions are propelled by an electrical potential into an analyzer. The ions enter the analyzer as an unsorted beam, and in the analyzer are sorted under the influence of an electro-magnetic iield in accordance with their specic mass into a series of divergent homogeneous ion beams. The sorted ions are collected anddischarged, the quantity of each kind of ions being measured by the amount of current that they discharge upon collection.

In one type of mass spectrometer, ionization is accomplished by bombarding molecules of the sample in an ionization chamber with an electron beam. The resulting ions are forced through a plurality of collimating electrodes having oriented slits or apertures therein, and emerge into an analyzer tube as an unsorted beam. It has been proposed heretofore to form the ions Within the chamber and to propel them into the collimating aperture in the first collimating electrode by means of a potential maintained across the zone of ion formation by means of a pusher electrode. The beam thus formed emerges from the slit of the rst collimating electrode and is propelled to the slit vin the second collimating electrode by a potential maintained between the tWo and it has been proposed heretofore to aid collimation by one or more auxiliary or focussing electrodes maintained at a potential intermediate those of the collimating electrodes and disposed between the collimating electrodes but to one or both sides of the ion beam. In this fashion a transverse component of propelling potential may be produced and employed to vary the Width of beam and `to aid in collimation .by preventing ion paths in the beam from crossing over each other between the collimating electrodes. i

As the result of my investigations, I have discovered that improved `results are obtained with a` mass spectrometer if the pusher electrode is, ineifect, eliminated so that there is no substantial `potential vbehind the zone of electron bombardment in which the ions are formed, but instead the potential is applied to the ions immediately after they diffuse from the region oi their formation. Thus my invention contemplates in a mass spectrometer, the combination which comprises an ionization chamber, an in let electrode disposed within the chamber and having an inner end provided with a passage for admission of molecules into the chamber with a slot `transverse to the passage, a second electrode spaced from the inlet electrode, means for producing an electrical propelling potential between the electrodes in the ionization chamber, and `means for propelling an electron beam through the slot transverse to the passage. In such `an arrangement of apparatus the ions are formed .by electron bombardment in a region outside the iield yof major ion propelling potential and enter this field primarily as a result of dffusion. Moreoven since ionization occurs in the restricted space of the slot rather than in the larger space Within the ionization chamber, the electron bombardment and consequent ionization takes place at a point where the sample is relatively highly concentrated and a higher percentage of the enteringV molecules are given an opportunity to ionize. This `increases the amount of ion-s produced from a given sample and increases the effective size of the sample, thus increasing the accuracy and speed of analysis.

In the preferred form of my apparatus the slot in the inlet electrode is in line with the slit in the adjacent propelling electrode and preferably lies in a plane deiined by the slits in the collimating electrodes, i. e. the plurality of propelling electrodes employed to form the ion beam and project it into the analyzer tube. It has been found that this type of structure aids markedly in obtaining improved results. s

Further improvements in results can be obtained if the current of the electron beam passing through the slot is maintained substantially constant, .for example, as indicated at anelectrcn catcher at which the s electrons are discharged after passing through the slot. In the structure of the invention practically all of the ion beam formed in the ionization chamber passes through theslits 'into theanalyzer, sothat the amount of ions entering the analyzer is proportional to the total electron current as collected by the catcher. It has been found that some types of ions formed in a mass spectrometer tend to lodge upon Vdielectric surfaces within the apparatus and to build up static charges on these surfaces. The gradualiaccumulaition of such charges `from this and other sources affects the calibration of the apparatus by changing the focussing of ions in the beam. The effect is particularly marked at the insulator which is employed as a spacer between collimating or propelling electrodes, and l have discovered that focussing electrodes disposed between the collimating electrodes may serve a double yfunction if they are constructed in the form of a shield which obstructs a major portion of the view between the ion beam and the inn sulator. In this manner the effect of the static charge of the insulator upon the ion beam is minimized. Accordingly my invention also contemplates the shielding of the ion beam from the effect of static charges built up on insulating surfaces by the interposition of a conductive shield therebetween which is electrically connected to a part of the apparatus maintained under potential and particularly the combination which comprises collimating electrodes, each having apertures through which an ion beam is propelled, an insulating spacer holding the electrodes apart, a plurality of focussing electrodes disposed respectively on opposite sides of the electron beam between the collimating electrodes and between the insulator and the beam, these focussing electrodes being spaced from each other and from the collimating electrodes and the insulator but of such size that they obstruct a major portion of the view between the insulator and .Y

the beam.

The foregoing type of structure is of particular benefit when employed in conjunction with the slotted inlet electrode described previously. Preferably, the focusing electrodes are plates bent at the two ends into channel shape and held in place (out of contact with each other and between the collimating electrodes) by leads projecting through the insulator. Thus the focussing electrodes substantially enclose the ion beam or ribbon passing through the slits.

The invention will be understood more thoroughly in the light of the following detailed description, taken in conjunction with the accompanying drawings in which:

Figs. 1 and 2 are longitudinal sections taken at right angles to each other through the head of a mass spectrometer adapted to the practice of my invention; and

Fig. 3 is a schematic diagram of a mass spectrometer equipped with the head of Figs. 1 and 2.

Referring to the drawings, it will be seen that the apparatus comprises a cylindrical head Ill the rear of which connects with a `gas inlet tube il of insulating material. Within the head there is an inlet electrode l2 in the form of a metallic tube which communicates with the gas inlet conduit at its rear and which is partially closed at its front end by a cylindrical velectrode plate I3 having a transverse slot to accommodate an electron beam I4.

The electron beam is produced by an electron gun I6 mounted outside a block l1 which encloses an ionization chamber I'S Within which the slotted electrode plate is disposed. (See Figs. 1 and 2.)

The electron beam enters the ionization cham-- ber through an aperture i9 which is in line with the slot in the inlet electrode, passes through the slot, and thence through an aperture in the opposite side of the block to an electron catcher 2i.

The front of the ionization chamber is formed by a propelling electrode 22. (composed of segments 22X, 22Y) provided with a slit SI1 that is substantially in line with the slot in the inlet electrode but slightly offset therefrom, since (due to the magnetic field) the ion paths are cycloidal. A second propelling electrode 23 provided with a slit S2 is disposed in front of the first propelling electrode with the slit S2 in line with the slot in the inlet electrode and the slit S1. The two propelling electrodes are spaced from each other by a rirg 24 of quartz or similar insulating material within which is a focussing chamber '25 containing an opposed pair of semicircular focussing electrodes 26, 2l which extend closeto, but are separated from, the two propelling electrodes by adequate dielectric gaps. The focussing electrodes are thus constructed to permit them to serve the dual function of focussing the ion beam passing through the slits S1, Sz and of shielding the beam from the effects of vagrant ions which tend to collecti upon the insulating spacer ring.

Electrode segments 22X, '22Y are spaced from each other by an insulator ring 22A through which, at a point spaced from its inner periphery, the segments are conductively joined tothe block ll' which forms the wall of the ionization chamber I8.

The ion beam is shielded from the effect of static charges built up on the insulator ring by a shield ring 22B disposed inside the insulator ring, electrically connected to the block, and separated from the electrode segments 22X, 22Y by a substantial gap.

The focussing chamber communicates with an analyzing chamber '28 of the mass spectrometer through the slit S2 in the second propelling electrode, which latter is fastened to an analyzer tube 29 that forms the wall of the analyzing chamber.

Referring now to Fig. 3, it will be observed that the head l of the mass spectrometer and the attached analyzer tube 29 are enclosed within an envelope 39, through th-e wall of which the gas inlet tube Il projects. A high degree of vacuum is maintained within the envelope by means of vacuum pumps (not shown) connected to the envelope.

The molecules of a gas sample being admitted into the ionization chamber through the inlet electrode are bombarded by the electron beam passing through the slot at its end and thus converted into ions. These ions are propelled as an unsorted beam into the analyzer tube by electrical potentials established between the several electrodes in the head by an electrical system shown schematically at the left of Fig. 3. Thus, a battery or other direct current supply fi is connected to the ends of a potentiometer di. The positive end of this potentiometer and its slider are connected through a switch 42 to a condenser 43.v A potential dividing network is connected across this condenser and takes the form of a resistor or potentiometer lli in series with a parallel pair of resistors or pou tentiometers all, 45A and a second pair 45, WSA which are in turn connected to ground at the negative end of the network.

The inlet electrode is connected to the positive side of thenetwork at the slider of the potentiometer 44. The two segments 22X, 22X of the first propelling electrode are connected respectively to the sliders of the potentiometers L55, 45A. One of the focussing electrodes is connected to the slider of the potentiometer 46; the other focussing electrode is connected to the slider of the potentiometer 46A, andthe second propelling electrode is connectedw'to the negative end of the network and to ground. The leads through which the foregoing connections `are .made pass through seals in the wall of the envelope. L

Inthe operation of the apparatus, a high degree of vacuum is established `withintheienvelope and a sample vof gas to 4be analyzed .is admitted through the gas inlet tubewand .passes through the slot in the inlet electrode, where it is bombarded by the electron beam. For optimum results the current producing this electron beam should be maintained constant, which result can be obtained by observing the current at the electron catcher by a conventional meter (not shown) and adjusting the electron current (for example, by the means shown in co-pending application Serial No. 591,010, filed April 30, 1945, by Harold W. Washburn et al.

Molecules of the gas sample are thus ionized and the resulting ions are propelled as an unsorted ion beam from the inlet electrode through the slots Si, S2 into the analyzer tube. There the unsorted beam is separated into a plurality of diverging homogeneous ion beams Bi, B2, B3, for example by means of an electro-magnetic eld produced in the analyzer tube by an electromagnet (not shown). Any one of the diverging beams may be focused on the exit slit at the end of the analyzer tube, so that it passes therethrough and strikes an ion collector. The current thus established in the ion collector is arnpliled and recorded by suitable apparatus and serves as an index of the quantity of that particular type of ions produced from the sample.

The position of the ion beams with respect to the exit slit (i. e. the focussing) may be changed by varying the electro-magnetic eld in the analyzer tube or by varying the propelling potentials. The latter method is preferred and may be practiced as follows:

The switch t2 in the energizing network is closed and the condenser 43 is charged to an appropriate level. Then the switch is opened and the condenser is permitted to discharge through the potential dividing network and the several electrodes in the head of the mass spectrometer. In this way the propelling potentials between the several electrodes gradually decay, while maintaining relative values.

It will be observed, however, that the potentials may be adjusted by suitably setting the potentiometers. Thus the potential at the inlet electrode may be adjusted by varying the setting of the potentiometer 44. Likewise, the potentials at the two propelling electrode segments 22X, 22Y can be varied by varying the setting of the potentiometers 45, 45A, and the potentials of the focussing electrodes 26, 21 may be varied by moving the sliders of the potentiometers 46A, 46. In this way the ion beams may be adjusted to give optimum results at the exit slit and ion collector. Thus by varying the potentials on the electrodes which define the slit Si, the position of the ion beam may be shifted laterally with respect to the slit and may be centered in the slit. By varying the potentials on the electrodes 26, 2l the ions may be brought to a focus in the slit S2 and also the focus may be moved laterally with respect to this slit.

I claim:

l. In a mass spectrometer, the combination which comprises an ionization chamber, an inlet electrode disposed within the chamber and having an inner end provided with a passage for Vadinissicvm of molecules into fthe chamber, the end 'having a -`slot transverse to the passage, a

second electrode spaced from the inlet electrode, means for producing an electrical propelling potential between the electrodes in the ionization chamber, and means for propelling an electron beam through the slot transverse to the passage.

2. In a mass spectrometer, the combination which comprises an ionization chamber, an inlet electrode disposed within the chamber and having an inner end provided with a passage for admission of molecules into the chamber, the end having a slot transverse to the passage, a second electrode spaced from the inlet electrode, and having a slit therein in line with the slot in the inlet electrode, means for propelling an electron beam through the slot transverse to the passage for forming ions therein, and means for producing an electrical propelling potential between the electrodes in the ionization chamber to force ions formed in the slot through the slit in the second electrode.

3. In a mass spectrometer, the combination which comprises an inlet electrode having an end provided with a passage for molecules, the end having a slot transverse to the passage, means for propelling an electron beam through the slot transverse to the passage to ionize molecules issuing therefrom, collimating electrodes each having an aperture therein through which an ion beam is propelled from the inlet electrode, an insulating spacer holding the collimating electrodes apart, a plurality of focussing electrodes disposed respectively on opposite sides of the ion beam between the collimating electrodes and between the insulator and the beam, these focussing electrodes being spaced from each other and from the collimating electrodes and the insulator but of such size that they obstruct a major portion of the view between the insulator and the beam.

4. Apparatus according to claim 3 in which the apertures in the collimating electrodes are slits in line with the slot in the inlet electrode.

5. In mass spectrometry involving the propulsion of ions from a nrst electrode to another by an electrical potential established therebetween, the improvement which comprises forming the ions by bombarding a relatively concentrated stream of molecules with electrons in a space within the rst electrode and permitting these ions to pass into the space between the electrodes by diffusion.

6. In mass spectrometry involving the propulsion of ions from a rst electrode to a second electrode by an electrical eld, the improvement which comprises forming the ions by electron bombardment of molecules in restricted space within the first electrode and permitting them to escape therefrom into the eld by diusion.

7. In mass spectrometry involving the collimation of ions by establishing an electrical eld between electrodes provided with orifices through which the ions pass, the improvement which comprises forming the ions by electron bombardment immediately prior to their introduction to the field, causing the ions to enter the eld by diffusion, and imposing on the ions a potential having a vector transverse to a line drawn from one orifice to another.

8. In a mass spectrometer having an ionization chamber and a first propelling electrode spaced from the ionization chamber by a spacing ring of insulating material and electrically connected to the ionization chamber through the ring at points spaced from the inner Wall thereof, the

improvement which comprises a conductive shield ring disposed around the inner Wall of the spacing ring, overlapping the inner wall of the ioniza- Jsion chamber and spaced from the electrode.

HAROLD W. WASHBURN.

REFERENCES CITED The following referenlces are of record in the file of this patent: Y

VUNlTED STATES PATENTS Name Date y Hpple July 18, 1944 OTHER REFERENCES Tech. publication, The Mass-Spectrograph and its Uses by Waiter Bleakney in American Physics Teacher, V01. 4, Feb. V1936, pages 12 to 23. Photostatic copy in Div. 36, U. S. Patent Omce,

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